1. Field of the Invention
The present invention generally relates to a scanning probe microscope such as a scanning tunnel microscope and an atomic force microscope and, more particularly, to a servo circuit for keeping a constant distance between a probe of the scanning probe microscope and a sample.
2. Description of the Related Art
A scanning probe microscope such as a scanning tunnel microscope (STM), an atomic force microscope (AFM), and a magnetic force microscope (MFM) has recently been used as a microscope having resolution on an atomic scale. The scanning probe microscope is disclosed in Elings et al., "Feedback Control for Scanning Tunnel Microscopes," U.S. Pat. No. 4,889,988 and in Elings et al., "Method to Increase the Speed of a Scanning Probe Microscope," U.S. Pat. No. 4,954,704.
A probe whose tip has a radius of about 100 nm, is brought close to a conductive sample at a distance of 1 nm. If a voltage of several volts is applied between the probe and sample in this state, tunnel current of several nanoamperes flows between one of the atoms of the tip of the probe and one of the atoms of the sample which is the closest to the tip. The tunnel current reduces logarithmically as the distance between the probe and sample increases. The STM, which makes use of this phenomenon, causes the probe to scan the surface of the sample, and then outputs atomic arrangement of the surface of the sample as an image in accordance with an amount of the flowing tunnel current.
In the STM, generally, a piezoelectric device for driving the sample or probe is controlled by the output of a tunnel current servo circuit to keep the distance between the sample and probe constant in accordance with irregularities of the surface of the sample when the probe scans the surface of the sample.
At the beginning of development of the STM, its feature was that atomic arrangement could be observed in a sample having a simple atomic structure such as carbon and silicon. The servo technique of the STM allows a three-dimensional image to be formed from the pit structure of an optical disk serving as a sample having a nm-scale structure. The servo technique has an advantage which cannot be obtained from optical microscopes or electron microscopes.
There is a great demand that a scanning range of a probe be widened up to several square micrometers in a sample having a nm-scale structure. Further, the probe is greatly moved in its up-and-down direction (Z direction).
If, however, time required for forming one image of an STM is several tens of seconds which is equal to that required for forming that of a conventional STM, a relative speed of the probe of the STM has to be several tens of times as high as that of the probe of the conventional STM. To correctly respond to the irregularities of the surface of the sample, the cut-off frequency of the response of the servo circuit has to be heightened.
If there is a change in the movement of the probe in the up-and-down direction or there are great undulations on the sample, a high-speed response is required. If the high-speed response is not performed, the probe will collide with the undulations on the sample. Since the piezoelectric device used in a mechanism for slightly moving the probe of the STM has its natural frequency, if the band of a servo gain is easily broadened, the piezoelectric device resonates and the servo circuit will be made unstable.
The above is true of all scanning probe microscopes such as the AFM and MFM.